Method and apparatus for mobility management

ABSTRACT

A method of operating a target base station in a wireless communication network to determine an addressing information for a source base station. A method to operate a target base station in a wireless communication network includes receiving a connection request message from a terminal, transmitting, to the terminal, a terminal information request message, and receiving, from the terminal, a terminal information response message including source base station information in response to the terminal information request message.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

The present application is a continuation of U.S. application Ser. No.14/435,142, filed Apr. 10, 2015, which is a 371 National Stage ofInternational Application No. PCT/KR2013/006497, filed Jul. 19, 2013,which claims the benefit of United Kingdom Application No. GB 1218392.7,filed Oct. 12, 2012, the disclosures of each are incorporated herein byreference into the present disclosure as if fully set forth herein.

TECHNICAL FIELD

The present invention relates generally to wireless networks, and morespecifically to re-establishing connections between user equipment andbase stations in a wireless network.

BACKGROUND ART

Wireless mobile networks, in which a user equipment (UE) such as amobile handset communicates via wireless links to a network of basestations or other wireless access points connected to atelecommunications network, have undergone rapid development through anumber of generations. The initial deployment of systems using analoguesignalling has been superseded by second generation (2G) digital systemssuch as GSM (Global System for Mobile communications), which typicallyuse a radio access technology known as GERA (GSM Enhanced Data rates forGSM Evolution Radio Access), combined with an improved core network.

The second generation systems have themselves been replaced by oraugmented by third generation (3G) digital systems such as UMTS(Universal Mobile Telecommunications System), using the UTRA (UniversalTerrestrial Radio Access) radio access technology and a similar corenetwork to GSM. Third generation standards provide for a greaterthroughput of data than is provided by second generation systems, andthis trend is continued with the proposals by the Third GenerationPartnership Project (3GPP) of a new 4G system known as the evolvedpacket system (EPS) but more commonly referred to as the Long TermEvolution (LTE) system. LTE systems use an improved radio accesstechnology known as E-UTRA (Evolved UTRA), which offers potentiallygreater capacity and additional features compared with the previousstandards, combined with an improved core network technology referred toas the evolved packet core (EPC).

Note that the term “GERA” is used herein to refer to the radio accesstechnology associated with GERAN (GERA networks), “UTRA” is used torefer to the radio access technology associated with UTRAN (UTRAnetworks), and similarly the term “E-UTRA” is used to refer to the radioaccess technology associated with E-UTRAN (E-UTRA networks).

As in earlier wireless mobile standards, LTE is designed as a cellularsystem in which base stations, known as eNBs, provide coverage over oneor more cells. A mobile terminal in LTE, known as the user equipment(UE), communicates with just one base station and one cell at a time.The mobile terminal can exist in one of two communication states in LTE:an IDLE state in which the mobile terminal is basically on standby, anda CONNECTED state in which the mobile terminal is active.

In the IDLE state in LTE, the mobile terminal is tracked by the networkto a specific tracking area, which may cover several base stations. Themobile terminal is not assigned to any particular base station but mayitself choose which base station or base stations it listens to. Themain aim in this state is to minimise signalling and resources, andthereby maximise standby time for terminals with limited battery power.

In contrast, in the CONNECTED state in LTE, the mobile terminal has aserving base station allocated to it, has its location tracked to theserving base station, and has active bearers which allow the terminal totransmit and receive at relatively high data rates. Accordingly, theterminal is actively managed by the network as it moves across atracking area or beyond. Handover is the normal mobility mechanism inthe CONNECTED state, and handles the smooth, planned transition of theterminal's connections as the terminal moves from one cell to another,and from one serving base station to another.

In LTE, the Handover mechanism is only available when the terminal is inthe CONNECTED state. In the IDLE state, the terminal is free to movewithin the tracking area and so follows a less rigorous Cell Reselectionmobility mechanism.

The Handover mechanism in LTE is managed by the network. That is, thenetwork side determines, based on measurement data received from theuser equipment, which destination cell and associated destination basestation the user equipment should connect to as it moves out of range ofthe serving base station. From the point of view of the terminal in aHandover, the destination base station is often referred to as thetarget base station, while the serving base station the terminal ismoving away from is referred to as the source base station.

In general, the Handover sequence involves the source base station ornode initially deciding that a handover should occur, and to whichtarget base station. The source base station then prepares the targetbase station for the handover by providing relevant connectioninformation associated with the specific user equipment. Once prepared,the target base station acknowledges this to the source base station,and the source base station commands the user equipment to handover tothe target base station. In response, the user equipment detaches fromthe source base station, and synchronises to the designated new targetcell.

If a failure occurs in the mobility mechanism in the CONNECTED state,the terminal can lose its tight allocation to a serving base station. Inthis situation, there is a risk that the terminal may have to return tothe IDLE state with an associated disruption to the data connections andpotential data loss. This disruption of the connection to the sourcebase station may be the result of a Radio Link Failure (RLF).

In order to improve the robustness of the terminal CONNECTED state, theLTE system prescribes a second mobility mechanism known asRe-establishment. Re-establishment is designed to maintain the CONNECTEDstate, and avoid unintended returns to the IDLE state. TheRe-establishment mechanism is used to recover from several errorsituations in which the network failed to execute a handover in time.The Re-establishment mechanism allows a user equipment that has lost itsconnection with a serving base station (and has not been commanded toHandover) to reconnect to a potential target base station. Aprecondition for the Re-establishment to be successful is that thetarget base station is aware of the connection settings from thepreviously disrupted connection i.e. the target base station hasinformation on the so-called UE context.

However, if there is an error in the Handover mechanism, then there is ahigh likelihood that the target base station will not be prepared, andwill not have the relevant up-to-date connection information for theterminal to recover while in the CONNECTED state. This limits theeffectiveness of the Re-establishment mechanism.

One conceivable solution to this limitation is to prepare one or moreneighbouring base stations in anticipation of any potential radio linkfailure. However, this is difficult to achieve without creatingsignificant additional signalling within the E-UTRAN. The number of basestations that would need to be prepared could be significant dependingon the size and topography of the cells, and those base stations thatare prepared are likely to need constant updating to ensure the UEcontext information is up-to-date.

SUMMARY

There is a need of a technique to improve the robustness of the terminalconnected state.

In accordance with a first aspect of the present invention, there isprovided a method of operating a target base station in a wirelesscommunication network to re-establish a connection with a mobileterminal, the mobile terminal having an earlier disrupted connectionwith a source base station of the mobile communication network, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: receiving a connectionrequest from the mobile terminal to the target base station comprising acell identifier for a cell served by the source base station, and aconnected equipment identifier for the mobile terminal, processing theconnection request to determine an addressing information for the sourcebase station using information contained in both the cell identifier andthe connected equipment identifier, and sending a request for theconfiguration information to the source base station using thedetermined addressing information.

An advantage of re-establishing a connection in accordance with thefirst aspect of the invention is that the source base station can bemore precisely identified. This, in turn, increases the chances ofreceiving accurate and relevant configuration information at the targetbase station for building up the connection with the mobile terminalaccording to a re-establishment mechanism.

In an embodiment of the invention, the connected equipment identifiercontains information which can be added to the cell identifier toprovide a more specific identification of the cell served by the sourcebase station. Because the identification of the cell is more specific,the target base station is less likely to request the configurationinformation erroneously from another base station serving a cell withthe same cell identifier as the source base station.

In an embodiment of the invention, the connected equipment identifiercontains information identifying a group of cells, whereby the group ofcells includes the cell served by the source base station. Preferably,the addressing information comprises routing information to the group ofcells. By providing routing information to a group of cells, the requestfor configuration information can be directed to a specific part of thenetwork where the cell identifier is more likely to be unique to aspecific cell. The specific part of the network may, for example, be anetwork entity associated with the group of cells, and the routinginformation would then enable the configuration information request tobe sent via that network entity. Possible network entities include apico-cell gateway or a home base station gateway.

In an embodiment of the invention, the connected equipment identifiercomprises a first subset of information for assisting with uniquelyidentifying the cell served by the source base station, and a secondsubset of information for assisting with uniquely identifying the mobileterminal served by the source base station. The connected equipmentidentifier may be a data field with a predetermined number of bits, andthe first subset of information may be one or more bits of the datafield.

Preferably, the number of bits used for the first subset of informationis configured consistently across a plurality of cells. This has theadvantage that the target base station can process the first subset ofinformation in the same way in relation to mobile terminals originatingfrom a plurality of source base stations. The source base stations canbe configured to use the same number of bits by means of networkinterfaces connecting the multiple base stations. For example, thenetwork interfaces could be direct interfaces between neighbouringcells. Alternatively, the network interfaces used to configure thenumber of bits could be implemented via a network element in the corenetwork.

Suitably, the cell identifier of the source base station is a physicalcell identifier (PCI).

Typically, the earlier disrupted connection is a connection which hasexperienced a radio link failure.

In accordance with a second aspect of the present invention, there isprovided a method of re-establishing a connection between a mobileterminal and a target base station, the mobile terminal having anearlier disrupted connection in a cell served by a source base station,and the source base station containing configuration informationassociated with the disrupted connection, the method comprising: sendinga connection request from the mobile terminal to the target base stationcomprising a cell identifier for a cell served by the source basestation, and a connected equipment identifier for the mobile terminal,processing the connection request in the target base station todetermine addressing information for the source base station usinginformation contained in both the cell identifier and the connectedequipment identifier, and sending a request for the configurationinformation from the target base station to the source base stationusing the determined addressing information.

In accordance with a third aspect of the present invention, there isprovided a method of allocating connected equipment identifiers formobile terminals in a wireless communication network, wherein thenetwork allocates a first set of connected equipment identifiers forassignment to mobile terminals operating in a first serving cell in thenetwork, and a second set of connected equipment identifiers forassignment to mobile terminals operating in a second serving cell of thenetwork.

An advantage with allocating different sets of connected equipmentidentifiers with different cells in the network, is that, connectedequipment identifiers can then be used to assist later with identifyingthe cell to which they are allocated. For example, in a connectionre-establishment procedure, a target base station receiving informationfrom a mobile terminal about a previous connected equipment identifiercan associate the set to which connected equipment identifier belongs toa particular source base station. Hence, the source base station can bemore reliably identified.

In an embodiment of the invention, a mobile terminal that has beenassigned a connected equipment identifier in the first or second servingcell, may send a connection re-establishment request to a target basestation in a third serving cell, and the target base station maydetermine which of the first or second serving cells assigned theconnected equipment identifier based on whether the connected equipmentidentifier belongs to the first or second set of connected equipmentidentifiers.

In accordance with a fourth aspect of the present invention, there isprovided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: receive a connection request from a mobileterminal comprising a cell identifier for a cell served by the sourcebase station, and a connected equipment identifier for the mobileterminal, process the connection request to determine an addressinginformation for the source base station using information contained inboth the cell identifier and the connected equipment identifier, andsend a request for the configuration information to the source basestation using the determined addressing information.

In accordance with a fifth aspect of the present invention, there isprovided a method of establishing a connection between a mobile terminaland a target base station, the mobile terminal having an earlierdisrupted connection in a cell served by a source base station, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: sending a connectionrequest from the mobile terminal to the target base station comprising acell identifier for a cell served by the source base station, aconnected equipment identifier for the mobile terminal, and supplementalcell identifier information, processing the connection request in thetarget base station to add the supplemental cell identifier informationto the cell identifier to provide a more specific identification of thecell served by the source base station, in order to determine moreaccurate addressing information for the source base station, and sendinga request for the configuration information from the target base stationto the source base station using the determined addressing information.

In accordance with a sixth aspect of the present invention, there isprovided a method of establishing a connection between a mobile terminaland a target base station, the mobile terminal having an earlierdisrupted connection in a cell served by a source base station, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: sending a connectionrequest from the mobile terminal to the target base station comprising acell identifier for a cell served by the source base station, aconnected equipment identifier for the mobile terminal, and supplementalrouting information for identifying a group of cells including the cellserved by the source base station, processing the connection request inthe target base station to determine addressing information for thesource base station using information contained in both the cellidentifier and the supplemental routing information, and sending arequest for the configuration information from the target base stationto the source base station using the determined addressing information.

In accordance with a seventh aspect of the present invention, there isprovided a method of operating a target base station in a wirelesscommunication network to re-establish a connection between a mobileterminal and a target base station, the mobile terminal having anearlier disrupted connection in a cell served by a source base station,and the source base station containing configuration informationassociated with the disrupted connection, the method comprising:receiving a connection request from the mobile terminal comprising acell identifier for a cell served by the source base station, aconnected equipment identifier for the mobile terminal, and supplementalcell identifier information, processing the connection request in thetarget base station to add the supplemental cell identifier informationto the cell identifier to provide a more specific identification of thecell served by the source base station, in order to determine moreaccurate addressing information for the source base station, and sendinga request for the configuration information from the target base stationto the source base station using the determined addressing information.

In accordance with an eighth aspect of the present invention, there isprovided a method of operating a target base station in a wirelesscommunication network to re-establish a connection between a mobileterminal and a target base station, the mobile terminal having anearlier disrupted connection in a cell served by a source base station,and the source base station containing configuration informationassociated with the disrupted connection, the method comprising:receiving a connection request from the mobile terminal comprising acell identifier for a cell served by the source base station, aconnected equipment identifier for the mobile terminal, and supplementalrouting information for identifying a group of cells including the cellserved by the source base station, processing the connection request inthe target base station to determine addressing information for thesource base station using information contained in both the cellidentifier and the supplemental routing information, and sending arequest for the configuration information from the target base stationto the source base station using the determined addressing information.

In accordance with a ninth aspect of the present invention, there isprovided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: receive a connection request from the mobileterminal comprising a cell identifier for a cell served by the sourcebase station, a connected equipment identifier for the mobile terminal,and supplemental cell identifier information, process the connectionrequest to add the supplemental cell identifier information to the cellidentifier to provide a more specific identification of the cell servedby the source base station, in order to determine more accurateaddressing information for the source base station, and send a requestfor the configuration information from the target base station to thesource base station using the determined addressing information.

In accordance with a tenth aspect of the present invention, there isprovided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: receive a connection request from the mobileterminal comprising a cell identifier for a cell served by the sourcebase station, a connected equipment identifier for the mobile terminal,and supplemental routing information for identifying a group of cellsincluding the cell served by the source base station, process theconnection request in the target base station to determine addressinginformation for the source base station using information contained inboth the cell identifier and the supplemental routing information, andsend a request for the configuration information from the target basestation to the source base station using the determined addressinginformation.

In accordance with an eleventh aspect of the present invention, there isprovided a method of operating a target base station in a wirelesscommunication network to re-establish a connection with a mobileterminal, the mobile terminal having an earlier disrupted connectionwith a source base station of the mobile communication network, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: broadcasting, in aserving cell, an indicator from the target base station to indicate tomobile terminals in the serving cell that the base station is capable ofreceiving a first and a second type of connection re-establishmentrequest, and receiving a connection request from a mobile terminal inthe serving cell, and processing the request according to whether it isa first type of request or a second type of request.

An advantage of the target base station being able to process first andsecond types of connection re-establishment request, is that, the targetbase station can work with legacy connection re-establishment requesttypes in addition to new re-establishment request types. Preferably, thefirst type of connection re-establishment request comprises basic cellidentity information, and the second type of connection re-establishmentrequest comprises basic cell identity information and additional cellidentity information to more reliably determine the addressinginformation for the source base station. Suitably, the connectionre-establishment request is processed to determine addressinginformation for the source base station.

In an embodiment of the invention, the basic cell identity informationis a physical cell identifier (PCI), and the additional cell identityinformation is at least part of a cell ID (CI), a global cell ID (GCI),or a tracking area code (TAC).

In accordance with a twelfth aspect of the present invention, there isprovided a method of operating a mobile terminal to re-establish aconnection with a wireless communication network via a target basestation, the mobile terminal having an earlier disrupted connection witha source base station of the mobile communication network, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: determining if a targetbase station supports a first type of connection request based onreceipt of an indicator from the target base station, sending a firsttype of connection request if the target base station is determined tosupport the first type of connection request, and sending a second typeof connection request if the target base station is not determined tosupport the first type of connection request.

In accordance with this aspect of the invention, the first type ofconnection request can contain additional cell identificationinformation compared to the second type of connection request.

In accordance with a thirteenth aspect of the present invention, thereis provided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: broadcast, in a serving cell, an indicator toindicate to mobile terminals in the serving cell that the base stationis capable of receiving a first and a second type of connectionre-establishment request, and receive a connection request from a mobileterminal in the serving cell, and processing the request according towhether it is a first type of request or a second type of request.

In accordance with a fourteenth aspect of the present invention, thereis provided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: determine if a target base station supports afirst type of connection request based on receipt of an indicator fromthe target base station, send a first type of connection request if thetarget base station is determined to support the first type ofconnection request, and send a second type of connection request if thetarget base station is not determined to support the first type ofconnection request.

In accordance with a fifteenth aspect of the present invention, there isprovided a method of operating a target base station in a wirelesscommunication network to re-establish a connection with a mobileterminal, the mobile terminal having an earlier disrupted connectionwith a source base station of the mobile communication network, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: receiving a connectionrequest from a mobile terminal, determining from the connection requestif the mobile terminal is capable of responding to an informationrequest sent by the target base station, and sending an informationrequest to the mobile terminal if the mobile terminal is determined tobe capable of responding.

An advantage with determining if the mobile terminal can respond to aninformation request, is that, the target base station can work withlegacy mobile stations and the possible limitations in the connectionre-establishment requests. At the same time, the target base station cansupport mobile stations capable of providing additional information,such as information to assist with the re-establishment request.

According to one embodiment, the mobile terminal is determined to becapable of responding to an information request if one or more bits inthe received connection request indicate that the mobile terminal iscapable.

According to another embodiment, the mobile terminal is determined to becapable of responding to an information request if the format ofconnection request indicates that the mobile is capable.

The method may further comprise receiving a message in response to theinformation request comprising cell identification information of a cellserved by the source base station.

The method may also further comprise determining addressing informationfor the source base station based on the received cell identificationinformation.

In accordance with a sixteenth aspect of the present invention, there isprovided a method of operating a mobile terminal to re-establish aconnection with a wireless communication network via a target basestation, the mobile terminal having an earlier disrupted connection witha source base station of the mobile communication network, and thesource base station containing configuration information associated withthe disrupted connection, the method comprising: sending a connectionrequest to the target base station including an indication that themobile terminal is capable of responding to an information request fromthe target base station, receiving an information request from thetarget base station, and responding to the information request bysending further information to the target base station to assist withthe connection.

In accordance with a seventeenth aspect of the present invention, thereis provided a base station for operating in a wireless communicationnetwork for re-establishing connections with a mobile terminal as atarget base station wherein the mobile terminal has an earlier disruptedconnection with a source base station of the mobile communicationnetwork, and the source base station containing configurationinformation associated with the disrupted connection, the target basestation being arranged to: receive a connection request from a mobileterminal, determine from the connection request if the mobile terminalis capable of responding to an information request sent by the targetbase station, and send an information request to the mobile terminal ifthe mobile terminal is determined to be capable of responding.

In accordance with a eighteenth aspect of the present invention, thereis provided a mobile terminal for operating in a wireless communicationnetwork for re-establishing connections with target base stations, themobile terminal having an earlier disrupted connection with a sourcebase station of the mobile communication network, and the source basestation containing configuration information associated with thedisrupted connection, the mobile terminal being arranged to: send aconnection request to the target base station including an indicationthat the mobile terminal is capable of responding to an informationrequest from the target base station, receive an information requestfrom the target base station, and respond to the information request bysending further information to the target base station to assist withthe connection.

In accordance with a nineteenth aspect of the present invention, thereis provided a computer program comprising instructions such that whenthe computer program is executed on a computing device, the computingdevice is arranged to carry out any of the preceding methods.

There may be provided a non-transitory computer-readable storage mediumstoring a computer program as described above.

Further features and advantages of the invention will be apparent fromthe following description of preferred embodiments of the invention,which are given by way of example only.

According to the aspects of the present invention, the source basestation can be more precisely identified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams showing an X2-based Handovermechanism;

FIGS. 2A, 2B, and 2C are schematic diagrams showing an S1-based Handovermechanism;

FIG. 3 is a schematic diagram showing an X2-based Re-establishmentmechanism;

FIG. 4 is a schematic diagram illustrating an exemplary Re-establishmentrequest message;

FIG. 5 is a schematic diagram illustrating the distribution of threecells in a network;

FIG. 6 is a schematic diagram illustrating a PhysCellId and C-RNTIspace;

FIG. 7 is another schematic diagram illustrating a PhysCellId and C-RNTIspace;

FIG. 8 is a schematic diagram illustrating a Re-establishment mechanism;

FIG. 9 is another schematic diagram illustrating a Re-establishmentmechanism;

FIG. 10 is a flow diagram illustrating part of a Re-establishmentprocedure;

FIG. 11 is another flow diagram illustrating part of a Re-establishmentprocedure; and

FIG. 12 is a schematic diagram illustrating an typical LTE system;

DETAILED DESCRIPTION

By way of example, embodiments of the invention will now be described inthe context of an LTE (Long Term Evolution) cellular wirelesscommunications system operating in accordance with Release-10 and beyondof the LTE system standards. However, it will be understood that this isby way of example only and that other embodiments may involve otherwireless systems, operating to other releases and other standards.

A typical LTE cellular wireless communications system is illustratedschematically in FIG. 12. The LTE system comprises a core network, aradio access network, one or more mobile terminals, and ideally aninterface to external networks such as the internet or private corporatenetworks. The radio access network is known as the evolved UMTSterrestrial radio access network (E-UTRAN) in LTE, and handlescommunications between mobile terminals, known as user equipment (UEs)in LTE, and the core network, known as the evolved packet core (EPC) inLTE.

The E-UTRAN consists of a network of base stations known as evolved NodeBs (eNBs), labelled eNB1 and eNB2 in FIG. 12. Each eNB is a base stationthat serves and controls UEs in one or more cells. In FIG. 12, eNB1 isthe serving eNB for the cell 25 and the UE 10, while eNB2 is the servingeNB for the cell 35. The eNBs can communicate with each other over anoptional X2 interface, to the evolved packet core (EPC) via the Sinterface, and to the UEs over the radio interface. The evolved packetcore (EPC) includes a mobility management entity (MME) 40, as well as aserving gateway (S-GW) 50 and packet data network gateway (P-GW) 60. TheMME 40 handles high-level issues such as security, while the S-GW 50 andP-GW 60 are generally responsible for data traffic, including data to orfrom external networks.

Embodiments of the invention propose several enhancements related to howa base station (eNB) receiving a re-establishment request can identifythe last cell (eNB) that the mobile terminal (UE) was connected to i.e.before a problem situation arose and the re-establishment was triggered.

The following terminology is used in describing embodiments of theinvention:

Source cell (SCell): Cell where the UE was before the re-establishmentprocedure is triggered. Note that a typical example of a trigger wouldbe the occurrence of a Radio Link Failure (RLF).

Source eNB (SeNB): eNB handling the UE before the re-establishmentprocedure is triggered

Target cell (TCell): Cell where the UE attempts to re-establish, i.e. inwhich the UE sends the re-establishment request.

Target eNB (TeNB): eNB receiving the re-establishment request

As explained previously, LTE supports two main mechanisms by which UEmobility in CONNECTED mode is handle, namely:

1) Handover; and

2) Re-establishment

Handover is the normal CONNECTED mode mobility mechanism. There-establishment procedure is used to recover from several errorsituations in which the network failed to execute a handover in time.

However, as discussed, the re-establishment procedure has a limitation:the eNB towards which the re-establishment procedure is performed needsto be “prepared” i.e. needs to have received a “UE context” for theconcerning UE. Thus, in the current re-establishment mechanism, unlessthe eNB receiving the re-establishment request is “prepared”, there-establishment will most likely fail.

Handover

FIGS. 1A and 1B are an example of a Handover mechanism used in LTE inthe CONNECTED mode. The handover shown is called an “X2-handover” sincemessages between the Source-eNB and the Target-eNB are exchanged via theX2 interface. The time axis in FIGS. 1A and 1B is from top to bottom,with the signalling or message steps between appropriate networkelements shown horizontally. Arrows indicate the direction of thesignals or messages, and include bi-directional signals and messages. Ingeneral, boxes refer to decision or processing steps performed withinthe system.

Due to the relatively high data rates involved in the CONNECTED state,it is important for the network to carefully manage in which cell themobile should be communicating at any one time. The network systemachieves this by gathering periodic information about the signalstrengths within the network, and by making appropriate decisions onhandover between cells based on these signal strengths. Specifically,with reference to FIGS. 1A and 1B, the Source-eNB 20 at step 1.1 sends aMeasurement Control message to the UE 10. The UE performs signalmeasurements on the serving cell and its nearest neighbour, and at step1.2 sends Measurement Reports back to the Source-eNB 20.

A step 1.3, the Source-eNB 20 makes a decision, taking into account theinformation from the Measurement Reports, to handover the UE to aspecified neighbouring cell. In step 1.4, the Source-eNB 20 uses the X2interface to communicate a handover request to the Target eNB 30 servingthe specified neighbouring cell. The handover request asks the new basestation to take control of the mobile, and provides all the relevantconfiguration information related to the current connection so that thenew base station is prepared for arrival of the UE.

At step 1.5, the Target-eNB 30 decides which bearers from the existingconnection it is willing to accept. The Target-eNB then prepares an RRCConnection Reconfiguration message for the UE, and transmits this as anembedded message in a Handover Request Acknowledgement to the Source-eNBas shown in step 1.6. The Source-eNB in turn extracts the RRC ConnectionReconfiguration message, and forwards it to the UE 10 in step 1.7. Inresponse to receiving the RRC Connection Reconfiguration message, the UEdetaches from the old cell and synchronises to the new cell. The UE 10reconfigures itself to the Target eNB in steps 1.9 and 1.10. Oncereconfiguration has occurred, the UE can confirm that the connection tothe Target eNB is complete with the RRC Connection ReconnfirmationComplete message.

Variations on the above procedure exist depending on the topography ofthe old and new cells. For example, FIGS. 2A-2C are illustrates a secondhandover variant called the “S1-handover”. This alternative mechanism isused when the X2 interface is not present between Source-eNB and theTarget-eNB. In this procedure, messages are relayed via the MobilityManagement Entity (MME) using the S1 interface.

What is important to note about both the X2 and S1 handover sequences isthat before the UE comes to the Target-eNB (step 1.9 in the FIG. 1B, andstep 2.12 in FIG. 2B), the Target-eNB is already “prepared” and hasreceived the UE context (step 1.4 in FIG. 1A, and step 2.5 in FIG. 2A).

Re-Establishment

The re-establishment procedure in LTE was introduced to handle errorcases in which the network did not initiate a handover in time to theUE. As a result, the UE goes out of coverage of the existing cell(experiences Radio Link Failure (RLF)) and will attempt to re-establishthe connection in another cell.

The sequence for the Re-establishment procedure shares some of thecharacteristics of the Handover procedure. A significant difference isthat there is typically no command from the Source-eNB to the UE tohandover (step 1.7 in FIG. 1A). Instead, the UE just loses coverage inthe source cell. Following a quick cell selection procedure in the UE todetermine the appropriate Target cell, entry in the target cell isinitiated by a RRC Connection Re-establishment Request message from theUE to the Target-eNB.

The re-establishment procedure will only succeed if the new eNB to whichthe UE sends the re-establishment message has been “prepared”, i.e. hasreceived an up-to-date UE context. This UE context contains informationrelated to, for example, what services the UE is involved in, whattransport bearers the UE needs, and what radio configuration is usedtowards this UE and also contains security information. This informationis needed in order to allow any eNB to continue with the UE in CONNECTEDmode.

If the initial steps of an incomplete Handover procedure are performedbefore the UE experiences radio link failure then it is possible thatthe Target-eNB might have been sent the relevant UE context and wouldthus be “prepared”. However, for the Re-establishment procedure tosucceed, the UE should select a target cell, during the Cell Selectionprocedure, which is served by same Target-eNB previously prepared duringthe Handover.

Alternatively, a source eNB that detects one of its UEs entered badradio conditions could quickly trigger a handover preparation, not withthe purpose of actually performing a handover, but merely to ensurethat, for example, a neighbouring eNB is prepared and can handle apotential re-establishment request.

As discussed previously, it is not straightforward for a Source-eNB toanticipate a radio link failure, and in anticipation prepare other eNBs.For example, it is not known when a Source-eNB should perform such apreparation, or if a single eNB or multiple eNBs should be prepared.Note also that the UE context sent to the potential Target-eNB needs tobe up-to-date. In other words, if there is a significant change in theUE context, the previous handover preparation would have to be cancelledand a new one performed.

The specified procedures for Re-establishment in LTE all assume the UEcontext is pushed to the Target-eNB before the UE attempts theRe-establishment Request message at the Target-eNB. As an alternative,it might be desirable for a Target-eNB to pull the UE context fromSource-eNB after the UE attempts the Re-establishment Request message atthe Target-eNB.

Referring to FIG. 3, there is shown a proposed sequence for theRe-establishment procedure, in accordance with the invention, that makesuse of this type of pull mechanism for the UE context. Although FIG. 3shows an approach based on the X2 interface, it is also possible for theRe-establishment procedure to be based on an S1-based approach, in whichmessages 3.5 and 3.6 might be exchanged via the S1-interface.

The following steps are illustrated in FIG. 3:

-   -   Step 3.1. The UE sends RRC Connection Re-establishment Request        message during a contention based random access procedure;    -   Step 3.2. In order to make the UE consider the RACH procedure        completed, a Contention Resolution Identity MAC CE is        transmitted;    -   Step 3.3 The TeNB performs a first admission control step;    -   Step 3.4 The TeNB determines the unique identity of the SeNB        based on information received during step 3.1;    -   Step 3.5 The TeNB sends a Context Request message to the SeNB in        order to pull the configuration information from the SeNB;    -   Step 3.6 Upon receiving the Context Request, the SeNB retrieves        the relevant UE context/configuration information and sends it        in a Context Request Response message back to the TeNB;    -   Step 3.7 The TeNB can optionally perform a second admission        step;    -   Steps 3.10/3.11: The Re-establishment procedure is completed,        which results in radio bearer SRB1 being re-established, and        security reactivated without changing the algorithms;    -   Steps 3.12/3.13: The Reconfiguration procedure to also get        SRB2/DRBs up and running. This might also result in release of        non-accepted bearers (DRBs/ERABs) at the UE.    -   Steps 3.20/3.21/3.22/3.23: Path switch procedure. Note that the        UL data can already be handled in the TeNB as soon as the DRBs        are resumed in the TeNB if the SGW does not change. The DL data        can also be handled from that moment, although depending on        availability of forwarded data/data from SGW.    -   Step 3.25: The Re-establishment procedure is effectively        complete. Accordingly, the TeNB sends a UE Context Release        message to SeNB, confirming that the UE Context no longer needs        to be reserved at the SeNB and may be released.

Identifying Source eNB

In order for the context pull mechanism shown in steps 3.5 and 3.6 inFIG. 3 to work robustly, it is important for the TeNB to have accuratelyidentified the SeNB from which the UE originated in step 3.4. However,in step 3.1, the TeNB receives limited information from the UE to helpidentify the SeNB.

FIG. 4 illustrates the contents of the RRC Connection Re-establishmentRequest message from the LTE standard Release-10. The message comprisesthe following 5 fields:

-   -   a) a 9-bit physical cell identity (PhyCellId). This is the        physical layer identity of the previous cell the UE was        connected to.    -   b) a 16-bit identity of the UE in CONNECTED mode in the source        cell (C-RNTI? Cell Radio Network Temporary Identifier).    -   c) a 16-bit shortMAC-I used for security purposes    -   d) a 2-bit cause value to indicate what caused the        re-establishment e.g. handover failure.    -   e) a 2-bit spare field

The shortMAC-I can help the SeNB to verify that the UE computed a hashover the c-RNTI, PCI and CellIdentity (28 bits, unique in the PLMN). Inother words, it provides verification that the UE has the correctsecurity key KRRCint. The shortMAC-I serves no function in helping theTeNB to identify the SCell or SeNB.

The PhysCellId is an identification of the source cell. It is 9-bits insize and supports the possibility to distinguish between 503 differentcells.

The demands on capacity and increased mobile data rates within wirelessnetworks often leads to increased density of cells within a given area.Furthermore, heterogeneous LTE networks are beginning to evolve whichhave a plurality of pico cells overlapping with macro cells.

If a re-establishment procedure occurs within a network in which thetarget cell has neighbouring cells with the same physical cell identity(PhysCellId) then the Target eNB will not be able to precisely identifythe Source eNB. Hence, the likelihood of the re-establishment procedurefailing is increased.

For example, if the target cell is a large macro cell then it may haveupwards of 10,000 neighbouring pico cells. Clearly, some of theseneighbouring pico cells would have to share the same PhysCellId. Thismeans the Target-eNB would be unable to uniquely identify the Source eNBbased on the PhysCellId alone.

A similar drawback occurs in the Handover mechanism of FIG. 1A, where,in step 1.2, the UE reports a measurement for a physCellId of a cell inits neighbourhood. If the Target eNB receiving this measurement reportserved a macro cell with upwards of 10,000 neighbouring pico cells thenit would not know uniquely which of these neighbouring pico cells wasthe one that was measured.

In Handover, the term “PCI confusion” was used to identify thissituation in which the L1 identity (PCI=Physical Cell Identification),which is primarily targetted at providing a cell with a locally uniquecell identification in its coverage area, is not uniquely identifyingone cell to the eNB receiving the measurement.

An example of this is shown in FIG. 5. Here, cell 1 is the only cell inits local coverage area that is using PCI=89. However, when a UEoperating in cell 3 reports a PCI=89 measurement to the eNB of cell 3,then the coverage area of cell 3 becomes relevant. Cell 3 may be amacro-cell with a wide coverage area, and the eNB of cell 3 may be awareof both cell 1 and cell 2 as neighbouring cells. Hence, when the PCI=89measurement is received by the eNB of cell 3, it cannot know whether itis for cell 1 or cell 3.

In a pico cell, the number of UEs requiring a UE context in parallelmight be relatively low. In contrast, the number of pico cells that needto be uniquely identified by a Target eNB in a re-establishment scenariomay be relatively high. The signal space allocated in LTE for the PCIand the C-RNTI is fixed at 9-bits and 16-bits respectively. Thus, inaccordance with the invention, it has been observed that there may besituations where the information space allocated for the PCI is toosmall whilst at the same time there is redundancy in the C-RNTI space.

Being a 16-bit space, the C-RNTI can potentially assign up to 65,536unique identifier values for the UE contexts. A pico cell eNB mayrequire less than 256 unique identifier values for the UE contexts itmanages (including active UEs with a RRC connection, as well as UEs forwhich the context is still stored to handle potential future RLFreporting). Therefore, only 8-bits of the 16-bit C-RNTI space arerequired. Also, in a macro cell, it is generally unlikely that all65,536 unique identifiers values for C-RNTI are needed. If, for example,only 32,678 or less UE contexts need to be identified, then only amaximum of 15 of the 16 bits in the C-RNTI space are required.

Based on this observation, in a first solution for addressing thelimitations of identifying the Source eNB in a re-establishmentprocedure, it is possible for one of more bits of the C-RNTI space to beused for a different purpose. In particular, the redundant space inC-RNTI can be used for assisting with identification of the Source eNBin a re-establishment procedure.

The part of the C-RNTI space that is not necessary for UE contextidentification could be used in a variety of ways to enhance the TeNBpossibilities for identifying the SCell and routing messages to thecorrect SeNB. Two possible ways are explained, as follows, withreference to FIGS. 6 and 7.

A) As Extension for Cell Identification—See FIG. 6

By coordinating the C-RNTI space amongst pico cells, it would bepossible to give 256 cells each a unique C-RNTI space for 256 UE's. Inother words, each cell (of the 256 cells) is allocated a set of 256values in the 16-bit space that are unique to that cell.

Referring to FIG. 6, the 8 highest most significant bits of the C-RNTIspace are used as an extension of the Physical Cell Identification.

As a specific example, assume that the cell 1 and cell 2 in FIG. 5 havebeen allocated two different C-RNTI ranges. Cell 1 may be allocated arange of 256 C-RNTI values from 0x0500 to 0x05FF, and cell 2 may beallocated a C-RNTI range of 0x0600 to 0x06FF. Each cell would therebyonly be able to establish 256 UE contexts rather than the usual 65,536.Then, if cell 3 receives a re-establishment request from a UE reportingthat the previous PCI was 89 and the previous C-RNTI was 0x0628, the eNBof cell 3, being aware of the allocation of C-RNTI values, can concludethat the UE was previously served by cell 2 rather than cell 1, and thusshould obtain the UE context from the eNB serving cell 2.

By giving a different C-RNTI space to cells with the same PCI, thecombination of PCI and the additional cell identification informationfrom the C-RNTI can effectively provide 504*256=129,024 unique cellidentifications. This should be sufficient for identifying any pico cellin a macro cell.

Although, the example above refers to using 8-bits of the C-RNTI forcell identification purposes, it is in theory possible to use othernumbers of bits for cell identification purposes. The specific number ofbits used may vary depending on how the network is deployed, or how itevolves. Furthermore, it may be possible to allocate multiple ranges ofC-RNTI values to a particular cell where the where the number ofparallel UE contexts is expected to be higher. Further still, the rangeof C-RNTI values could be allocated to the different cells in adifferent manner—for example, the even bits used for the cellidentification and the odd bits used for UE context identification.

B) As New Routing Information—See FIG. 7

Alternatively, the bits available from the C-RNTI can be used to containa new separate information element that would ease the routing in theTeNB (i.e. not be combined with the PCI).

Using a similar 8-bit C-RNTI split from the first example, 8 bits wouldbe available in the C-RNTI to assist with addressing the Source-eNB. The8 highest most significant bits could be used to identify a pico-cellgateway (GW) or some other group of cells. As long as there is a centralentity to route to for that group of cells, and within that group ofcells the PCI is unique, still a unique cell identification can beobtained. The exact benefit of this approach depends on the grouping ofpico cells to pico-cell gateways. However, the allocation of PCIs withina network could be optimised to account for the pico-cell gatewaytopography. In any event, the likelihood of two identified Source cellshaving the same PCI value is decreased with this solution.

Routing might be simpler in this type of approach since less individualidentities have to be known in the TeNB. The group identifier could, forexample, contain part of the Global eNB-ID. For example, if all picoeNBs under a macro have only the last 8 bits of the Global eNB-IDdifferent (Tracking Area Identity (TAI), and the rest of the GlobaleNB-ID bits the same), then the macro eNB would only have to store oneset of the TAI plus the most significant bits of the eNB-ID and combinethis with the 8 bits received to complete the routing information in thecontext retrieval message.

A benefit of this solution is that there is no requirement to modifyexisting UEs that comply with the LTE standard. In other words, thesplitting of the C-RNTI space amongst different pico cells would betransparent for the UE. According to the Re-establishment procedureshown in FIG. 3, the UE just reports the C-RNTI it was allocated in theprevious cell.

The splitting of the C-RNTI space across different cells itself might becoordinated by the network operator (OAM). Furthermore, signalling canbe developed to coordinate the allocation of C-RNTI space over the X2interface, or the S1 interface.

In a second solution for addressing the limitations of identifying theSource eNB in a re-establishment procedure, it has been observed thatthe 2 spare bits that are currently unused in the ReestablishmentRequest message can be employed to assist with identifying the SourceeNB.

Instead of using part of the C-RNTI space, the remaining 2 bits in there-establishment request could be used in combination with the PhysicalCell Identification to more reliably identify the Source eNB. Forexample, the 2 bits could be used as an extension of the cellidentification or to identify some group of cells in the mannerdescribed above.

This solution would avoid the C-RNTI space coordination requirement, butwould instead use the last 2 remaining bits in the re-establishmentrequest. Using this solution as an extension of the Physical Cell Id, itwould be possible to extend the cell identification space to 4*504=2016cells at the cost of using the last 2 remaining spare bits.

In a third solution for addressing the limitations of identifying theSource eNB in a re-establishment procedure, it has been observed thatthe re-establishment request procedure in LTE could be extended toenable messages from the UE that include routing identities such as thecell identifier (CI), Global Cell Identifier (GCI) and/or the TrackingArea Identifier (TAI). Specifically, the radio access network (E-UTRAN)can be adapted to handle a new re-establishment request message. Toensure backwards compatibility, the E-UTRAN can also support existingre-establishment request messages of the type shown in FIG. 4.

In order to support both message types (the existing and extendedre-establishment request messages), the network can indicate to UEs thatit supports the extended reestablishment request message. Also, the RACHprocedure should allow sending of a larger reestablishment requestmessage by the UE. This can be done by configuring the“messageSizeGroupA” parameter and setting it to a value equal to orlarger than the size of the contents of the extended re-establishmentrequest message with the extended routing information.

The third solution is illustrated in the steps of the flowchart of FIG.10.

The extended re-establishment request message could alternatively be acompletely new type of re-establishment request message andcorresponding procedure. This variation on the third solution isillustrated in the steps of the flowchart of FIG. 11.

Note that it is beneficial to keep the message sizes at an optimal level(not unnecessary large) since increases in message size tend to have anadverse affect on the maximum cell coverage area.

In a fourth solution for addressing the limitations of identifying theSource eNB in a re-establishment procedure, it has been observed thatthe re-establishment request procedure in LTE could be extended toenable an additional request and response procedure between the TargeteNB and UE for obtaining additional information to more preciselyidentifying the Source eNB.

In order to support both procedures (the existing procedure under LTE,and the extended procedure including the request and responseprocedure), the Target eNB can be informed by the UE that the UEsupports the new request and response mechanism.

Specifically, the Target eNB can be informed of the supported newprocedure in the UE by using one of the spare bits in the current RRCConnection Reestablishment Request message (see FIG. 4). Alternatively,the UE can use a new format for the connection reestablishment requestmessage. In this case, the eNB can broadcast support for this type ofnew connection re-establishment message to inform the UE that it isallowed to use the new message.

If the TeNB recognises that the UE supports the request/responsemechanism, and it wants to obtain additional information from the UE, itcan execute the new request/response mechanism to obtain, for example,Source cell and/or Source eNB identification information. Note that thistype of request/response mechanism could also be used for obtainingother information from the UE such as the UE capability information.

FIG. 8 is an example of an extended re-establishment request procedurefor obtaining more accurate information on the Source eNB. FIG. 8follows the procedures of FIG. 3 but includes additional decision box8.2 a, and the request and response messages 8.2 b and 8.2 c.

In decision box 8.2 a, the Target eNB decides if it is able to determinethe identity of the Source eNB based on the information received in theRe-establishment request message in step 8.1. If the Target eNB decidesit needs additional information, it can check to see if the UE supportsan additional request-response procedure. Subsequently, in step 8.2 b,the Target eNB sends a new type of request for additional informationfrom the UE. In response, the UE can send additional cell identificationinformation of the Source eNB back to the Target eNB in step 8.2 c.

The request-response procedure in FIG. 8 can be a newly specifiedprocedure or it may be based on existing procedures for retrievingadditional UE information known from Handover procedures in LTE.

The re-establishment procedure and the subsequent reconfigurationprocedure in FIG. 8 could be combined into one procedure which makes allnecessary configurations for the UE and thus allows the user traffic (onDRB's) to continue in “one-step”.

It should be clear that this type of one-step reestablishment procedurewill require a UE context to be present in the TeNB. Thus if in such asolution the TeNB has insufficient information to identify theSCell/SeNB, still a mechanism would be required to obtain thisinformation from the UE. An example sequence is shown in FIG. 9 wherethe existing UE information procedure is modified to obtain thenecessary additional information, followed by a new one-stepreestablishment procedure.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

What is claimed is:
 1. A method to operate a terminal in a wirelesscommunication network, the method comprising: transmitting a connectionrequest message to a target base station, wherein the connection requestmessage is a radio resource control (RRC) connection reestablishmentrequest message; receiving, from the target base station, a terminalinformation request message including information indicating that radiolink failure information is requested; and transmitting, to the targetbase station, a terminal information response message includinginformation on a source base station where radio link failure isdetected, in response to the terminal information request message,wherein the information on the source base station comprises a trackingarea code (TAC).
 2. The method of claim 1, wherein the connectionrequest message comprises information indicating whether the terminal iscapable of responding to the terminal information request message. 3.The method of claim 2, wherein the information comprises a format of theconnection request message indicating that the terminal is capable ofresponding, or one or more bits indicating that the terminal is capableof responding.
 4. A terminal, the terminal configured to: transmit aconnection request message to a target base station, wherein theconnection request message is a radio resource control (RRC) connectionreestablishment request message, receive, from the target base station,a terminal information request message including information indicatingthat radio link failure information is requested, and transmit, to thetarget base station, a terminal information response message includinginformation on a source base station where radio link failure isdetected, in response to the terminal information request message,wherein the information on the source base station comprises a trackingarea code (TAC).
 5. The terminal of claim 4, wherein the connectionrequest message comprises information indicating whether the terminal iscapable of responding to the terminal information request message. 6.The terminal of claim 5, wherein the information comprises a format ofthe connection request message indicating that the terminal is capableof responding, or one or more bits indicating that the terminal iscapable of responding.